old computer systems useds to have black backgrounds and orange or yellow text.
I thought it was simply because the monochrome cathode-ray tubes didn't have shades of gray: pixels were either lit or not, and the tube would last longer if only the letter pixels where lit than the opposite.
In fact, I remember one of the first virus that would set a letter permanently on the screen, to the point it would burn the substrate and would damage the screen. I remember also the "falling letters" virus, in which the letters would fall as you were typing... Ah, those were the days, starting the PC with the MS DOS version 3 in a 5 1/4 floppy drive....
... and if you think about it, even in those days our SOHC's were already old....